1. Field Of The Invention
The present invention relates generally to digital data recording. More particularly, the invention relates to a method of and apparatus for detecting a predetermined indicium, such as an address mark, within a stream of information-bearing bits.
2. Description Relative To The Prior Art
In the recording of digital information on a record storage medium--magnetic tape, a disk (magnetic, magneto-optical, optical) or an equivalent thereof--bits recorded are systematically arranged for orderly retrieval. With a disk, for example, a record track is divided into a plurality of sectors, each of which contains a predetermined number of bits, which are commonly arranged into two groups. The first group of bits is known as a sector header and the second group, which follows the sector header, is known as a data field. A data field may further be divided into mini-sectors each of which contains a given number of bits.
The term "divided" is somewhat misleading since there may or may not be actual physical separation between two groups of bits. Rather, bits are recorded and/or played back as a continuous, uninterrupted bit stream, while the terms "sector header", "data field", and "mini-sector" merely serve as a convenient means of identifying respective portions of the bit stream.
Either the sector header or the data field, or both, commonly also includes bits corresponding to one or more address marks. An address mark functions advantageously to achieve byte (a set of eight binary bits) synchronization. In particular, an address mark serves for identifying a sector header or the leading edge of a stream of used data bits. Because of the importance of user data, multiple address marks within a given sector serve for redundantly providing byte synchronization.
A well known problem that must be handled in digital data recording is the need to detect an address mark in the stream of binary bits. It is known in the prior art to encode bits representing an address mark using a unique signal, either an MFM or a DMM pattern having one or more normal signal transitions missing. It is intended, of course, that such an address mark will not be mistaken for other information, such as user data, in the bit stream. U.S. Pats. Nos. 3,750,121, 4,319,287 and 4,752,841 disclose an address mark pattern that is different from any other pattern that results when a bit stream is encoded.
A surface defect in the recording medium, however, can cause one or more bits to be in error. Although an encoded address mark pattern may be different from any pattern for other encoded information, an uncorrected error may "look" like an address mark. If this were the case, signal processing apparatus would not be able to process playback data correctly.